@Lucky_Luxy here’s the response that Tom at MoneyBox has given me permission to post in the Monzo Community:
As you’ve already had funds taken from your original Direct Debit account, there are two ways you can change your Direct Debit - the Manual Change or the Auto Change. See below:
The first step will be to cancel your current Direct Debit with your bank. This will prevent any further money from being debited from that account.
We will require evidence that both your original account, and the account to which you wish to move your Direct Debit, both belong to you. We appreciate this is an inconvenience but as a financial services organisation, we are under strict fraud prevention and anti-money laundering obligations.
(a) We will need you to send us a recent bank statement for both your existing account and your new account. The easiest way is to send us a download of your statement from your online banking, saved in PDF and sent through to us here (the alternative would be to send us a photograph of the original paper statement) - in either case showing name, address, sort code and account number; and
(b) We will deposit a small amount of money in each account, each with a unique reference. You will just need to quote back to us the exact amounts and reference.
I will then be able to send you a Direct Debit update notification, which you will receive via the App, and we can get the DD change underway.
Re: Monzo current account - Monzo do not provide statements, so we’ve devised an agreement between ourselves; if you ask Monzo for written proof of account ownership for use with Moneybox, they should write you a letter confirming this. We’ll then be able to accept this in lieu of a bank statement. Just make sure it includes your account number and sort code, and we can make the required payments to confirm account ownership.
MoneyBox was receptive to my request and positive in proposing a solution in lieu of the awaited Monzo API.